In electronics manufacturing, solders provide inexpensive, mass-producible and generally reliable interconnections to complete the electronic circuitry between various elements that make up an electronic assembly. Solder joints provide electrical interconnections and serve as the mechanical attachment of the electronic components to the printed circuit board and also serve a critical heat transfer function as well. Soldering make these connections at temperatures just below those that would cause damage to some of the elements of the assembly and substrate materials.
Reflow soldering is the predominant soldering method for Surface Mount Technology (SMT) assemblies. This technique provides an opportunity to handle a wider range of electronic assemblies than were possible with wave soldering. Eutectic solder, 63% Sn/37% Pb solder (melting temperature being 183.degree. C.) is commonly used for reflow soldering with peak reflow temperature of 210.degree.-220.degree. C. In order to use low cost thermoplastic substrate materials and some temperature-sensitive components for integration of many of such components in a small area as perhaps attached to an automotive dashboard, it is required to reflow at lower peak temperatures than 220.degree. C.
It is known that eutectic 63 Sn/37 Pb solder joints are reliable during thermal cycling between -40.degree. C. and 125.degree. C. Since maximum temperature for thermal cycling in integration applications is 105.degree. C. instead of 125.degree. C., a low temperature solder based on tin-lead eutectic and having a melting temperature 20.degree. C. lower than that of 63 Sn/37 Pb is likely to survive thermal cycling between -40.degree. C. and 105.degree. C. Low temperature solder compositions such as 43 Sn/43 Pb/14 Bi or 52 Sn/48 Bi (U.S. Pat. No. 5,573,602) can be reflowed at a peak temperature of 180.degree. C. But these solder compositions have wide pasty ranges (up to 20.degree. C.) between solidus and liquidus temperatures which can lead to damage of the solder joints during reflow soldering/cooling process. Also, such solders which might include a significant amount of indium to reduce the melting point greatly increase the cost of the solder.
It would be desirable to provide new low temperature solders with a melting temperature of less than 165.degree. C. and with a narrow pasty range, preferably being about 5.degree. C., which can provide long term reliability of solder joints and which is priced to be commercially useful for large volume applications. The present invention provides such solder compositions.